The present invention relates to techniques for sensing conditions of a medium, such as paper or film, in a device performing operations on the medium. More specifically, the invention relates to techniques in which the amount of medium and malfunctions of a roll of medium are sensed based on the rotation of the medium roll.
In printing or plotting on a medium such as paper or film, failure to stop before the supply of medium is exhausted can damage the writing elements, necessitating expensive repairs. Therefore, means are conventionally provided in a printer to sense a no medium condition and protect the writing elements. Also, sensing a low medium condition makes it possible to delay a long plot or listing until an adequate medium supply is available.
One conventional technique for sensing low and no medium conditions depends on optically sensible marks placed on the non-printed side of the medium during manufacture. The sensing of a mark near the end of a medium roll indicates a no medium condition and the sensing of another, distinguishable mark some distance from the end indicates a low medium condition. This technique becomes unreliable, however, if the marks are incorrectly positioned, if the medium is damaged or torn, or if the medium is transparent or nearly transparent with black writing on its printed side which can be confused with a mark. Therefore, complicated backup media-out sensors are necessary. Also, powering off the printer after detection of the low medium mark will eliminate the low medium warning, the next warning occurring when the medium is exhausted.
Another conventional technique for sensing a low medium condition depends on a sensor which directly detects the radius of a roll of medium. U.S. Pat. No. 4,204,180, for example, discusses a device which mechanically detects the end of a paper roll in a printer, providing a signal indicating that a predetermined amount of paper remains. U.S. Pat. No. 4,239,404 discusses a system including an infrared transmitter and receiver arranged at opposite ends of a paper roll in a printer to detect a radius reflecting a low paper condition. The system also includes a mechanically deflected arm which detects a paper out condition and provides a signal. This is necessary because the radius of the medium core varies due to manufacturing tolerances, so that radius sensing can only detect low medium, not a no medium condition. In general, radius sensing depends on expensive mechanisms or circuitry requiring critical tolerances and adjustments.
A number of techniques are used in other fields for sensing or detecting the amount of an unwinding web-like material on a roll, to avoid reaching the roll end. U.S. Pat. No. 4,040,043 discusses apparatus for mechanically detecting the remaining quantity of film on a reel in a packaging apparatus. U.S. Pat. No. 4,491,430 and 4,213,575 discuss devices which mechanically monitor the amount of ribbon on a printer ribbon reel. U.S. Pat. No. 4,097,726 discusses a device which detects an approaching tape ending in a tape recorder by comparing the rate at which the tape roll rotates with a reference rate.
Some techniques for automatically splicing or exchanging rolls of web material rely on reaching a predetermined web length or roll radius before splicing or exchanging. U.S. Reissue Patent Re. 30,868 relates to a paper splicing apparatus in which a predetermined roll diameter is sensed by comparing two pulse trains, one resulting from the paper roll rotation and the other from a guide roll rotation; when the rates coincide, the diameter has been reached. U.S. Pat. No. 4,089,482 similarly measures the length of web unwound per revolution of a web roll and compares it to a predetermined length as part of determining when to splice. U.S. Pat. Nos. 4,337,903 and 4,151,403 also take into account both web length counting pulses and roll revolution counting pulses in controlling a roll exchange or splice. U.S. Pat. No. 4,021,002 relates to a different auto-splice technique using photosensors to detect radius while also measuring length.
Other techniques make use of web length and roll rotation measurements in controlling the winding or unwinding of a web-like material during a manufacturing process. U.S. Pat. No. 4,535,949 discusses a device for measuring rolled-up length of a web roll which, upon sensing a web break, calculates roll circumference based on web length measurement and web roll rotation counting. U.S. Pat. No. 4,463,913 describes prior art apparatus which measures circumferential length of a coil turn based on rotational rates of the coil and of a motor driving a bridle through which the coiled material passes. U.S. Pat. Nos. 4,159,572 and 3,898,436 discuss similar techniques relating rates of rotation.
It would be advantageous to provide a simple and inexpensive medium condition sensing technique for use in a printer or plotter which would not depend on marks on the medium or on the direct detection of roll radius. It would further be advantageous to provide a sensing technique which would operate properly despite the printing operation itself and despite malfunctions such as running out of medium or a medium break.